This invention relates to a digital data transmission system and more particularly to a system in which transmission of pulse data is effected through a buffer memory.
In PCM transmission systems, if digital data bits with logical "0" occur in succession, a phase jitter is caused in the timing signal when it is reproduced in a PCM repeater, causing errors in reproduced codes. In a prior art transmission system, a zero pattern inhibit circuit is employed to prevent the occurrence of the successive logical "0" bit pattern, in order to cope with the above-mentioned problem. In more detail, the inhibit circuit monitors, for example, each word of an input data bit string entering a PCM terminal unit, and when a word consisting of successive "0" bits is detected, a bit exhibiting the least weight within the word is forcibly changed from "0" to "1."
However, if such an inhibit circuit is applied to an inter-frame coding device handling signals removed of redundant bits, such as band compressed digital video signals, the picture quality will disadvantageously be deteriorated. In order to overcome this deficiency, the above device employs a forced zero-inserting circuit which inserts logical "1" at a fixed period into transmission data so as to prevent occurrence of a successive logical "0" bit pattern which continues over a specified number of bits. This forced zero-inserting circuit, however, forces insertion of the logical "1" bit at unwanted portions where the successive "0" bit pattern does not occur, thus degrading the data transmission efficiency.